Glass fiber reinforced plastics have been used widely in recent years in the automotive industry. These plastics include sheet molding compound, glass fiber reinforced reaction injection moldable materials and various other glass fiber reinforced plastics. The advantages of lightweight, high strength, rustproofing and relatively low cost make them ideal for many automotive interior and exterior body panel applications.
Of these materials, the reinforced reaction injection moldable material (RRIM) is of particular interest to the automotive industry since they can be processed economically with low cost equipment. RRIM materials filled with milled glass have greatly improved stiffness and are suitable for applications where structural integrity is required. These materials are also suitable for use in large automobile exterior body panels since they produce readily paintable surfaces. However, one inherent drawback of these RRIM materials is that they have a relatively high coefficient of thermal expansion, i.e., 33.times.10.sup.- /.degree.C. in the direction parallel to flow, 108.times.10.sup.-6 /.degree.C. in the direction perpendicular to flow, as compared to that of only 12.times.10.sup.-6 /.degree.C. for steel. Consequently, when used in relatively large and flat panels where the service life of the part covers a wide temperature span (e.g., -20.degree. C. to +70.degree. C.), the dimensional stability of the RRIM materials is often less than desirable.
To meet the requirement of dimensional stability on a large body panel, a new type of glass fiber reinforced RIM material was subsequently developed by using a glass fiber mat of continuous glass. The presence of a glass fiber mat embedded in a large RIM part through the whole area dramatically improves its dimensional stability, while all other desirable mechanical and processing properties are maintained.
The processing of glass fiber mat reinforced RIM is relatively simple. It involves placing a glass fiber mat in a mold cavity and shooting RIM material into the closed mold so that the RIM material is soaked through the mat. A completed part is then removed after it is cured in the mold.
A typical RIM used in this process is a polyurethane based material produced from two components: an isocyanate and a polyol. The in-mold pressure normally seen in the RIM process is less than 50 psi, requiring much less clamping force than other processes.
The glass mat reinforced RIM material is especially suitable for large body panels such as door panels or quarter panels on a vehicle. The dimensional stability of a resulting door panel or quarter panel is greatly improved while the traditional characteristics of RRIM, i.e., stiffness, strength, and ease of processing are maintained. The coefficient of thermal expansion of a glass mat reinforced RIM material is only one-third (13 .times.10.sup.-6 /.degree.C.) of that for a RRIM material filled with milled glass. As a matter of fact, its thermal stability property is even superior to that of aluminum.
In the processing of glass fiber mat reinforced RIM parts where aesthetic property is important, however, a new problem has arisen. This is the glass fiber readout problem observed in the surface layer of a glass mat reinforced RIM part. The readout problem is caused largely by the presence of the continuous glass fiber in the surface layer of the panel. When a panel is situated in a mold under compression, the resin material located between the panel surface and a glass fiber in the surface layer of the panel is under higher pressure than the resin material located not adjacent to a glass fiber. As a consequence, when the part is demolded, the cured viscoelastic resin material located adjacent to a glass fiber will expand more than the resin material not adjacent to a glass fiber. This results in a panel with a surface showing protruded contours of various glass fibers which are located immediately below the surface of the panel, commonly known as the glass fiber readout problem. Another cause of fiber readout is the differential thermal shrinkage that exists between the glass fibers and the resin. The coefficient of thermal expansion of polymers is at least one order of magnitude larger than the coefficient of thermal expansion for glass fibers. As a result, when a molded part is removed from the mold and allowed to cool to lower temperatures, the glass rich regions shrink less than the resin rich regions which in turn causes the formation of protuberances over the fibers near the surface.
Another type of glass fiber reinforced polymeric products that suffer from fiber readout is sheet molding compound (SMC) parts molded from charges with high mold coverage, i.e., with minimum flow. The reason for reducing the extent of flow in the mold is to overcome the problem of long term waviness. For that purpose SMC can be compounded with chopped roving or continuous glass fiber mats and the charge will be cut to the size of the cavity such that the paste does not flow considerably. The disadvantage of this method is the formation of fiber readout on the cosmetic surface.
Numerous efforts have been made to correct the glass fiber readout problem. However, none of them were found to work satisfactorily in hiding the glass readout on the surface of a RIM part. For instance, an in-mold coating process used on a conventional RIM part disclosed in U.S. Pat. No. 4,282,285 was found inadequate to correct the problem. This fiber readout problem makes the glass fiber mat reinforced RIM material unsuitable for exterior automobile body panel applications for aesthetic reasons.
I have previously disclosed a method of producing glass fiber mat reinforced plastic panels with smooth surfaces in U.S. Pat. No. 4,610,835. In that method, premolded glass fiber mat reinforced plastic panels are first cooled to room temperature and then coated with a room temperature curable polyurethane composition. A molding pressure is then applied momentarily to spread out the coating material and then released while the coating material is still in its fluid state. Even though the process generally works well in producing glass fiber mat reinforced plastic panels with a smooth surface, it has several processing drawbacks. First, the panels after the molding process must first be cooled to room temperature before the coating material can be applied. This means that storage facilities must be provided such that panels may be stored between the molding and the coating operation. Consequently, the same molding machine can not be used in the coating operation which leads to substantially higher manufacturing cost. Secondly, since only room temperature curable polyurethane composition can be used in that process, longer curing times are required.
It is therefore an object of the present invention to provide a method of producing glass mat reinforced panels with smooth surfaces without the fiber readout problem.
It is another object of the present invention to provide a method of making glass mat reinforced panels for automotive exterior body panel applications having smooth surfaces by coating premolded panels maintained at or near the panel molding temperatures with a polymeric coating material.
It is a further objective of the present invention to provide a method of producing glass mat reinforced panels for automotive exterior body panel applications having smooth and predecorated surfaces that do not require further painting or decorating.